The present invention relates to a system for transmitting sequences of digital samples encoded by variable-length binary words, the system being formed on the one hand by a transmitter device comprising a statistic encoding circuit for encoding in said variable-length words, words of a fixed length appearing at its input as a sequence of samples, a synchronizing circuit for forming synchronizing words which define the position of the variable-length words in the sequences and a multiplexer circuit for combining, in view of their transmission, the synchronizing words with the variable-length words, and, on the other hand, a receiver device comprising a demultiplexer circuit for applying from one of its two outputs the variable-length words received to a statistic decoding circuit for recovering the fixed-length words and from its other output the synchronizing word to a processing circuit for recovering, in the event of a transmission error, the position of the samples in the sequences.
Such a system is used with great advantage in more specifically the transmission, in digital form, of television pictures, each sample sequence then representing a line of a picture and the samples representing the picture elements (pixels).
U.S. Pat. No. 3,971,888 discloses a system of this type of said usage.
The problems encountered in this type of system is that it is sensitive to transmission errors. A transmission error affects in the first place the binary word containing this error and may have repercussions on the subsequent words; the result is that, although ultimately the words are received in the appropriate manner, they no longer correspond to the position of the sample in the sequence. This change in position propagates from line to line and then a picture of a significantly degraded quality is received. To solve this problem partially, said United States Patent recommends the transmission of synchronizing words at the beginning of the sequence and to protect them by an error correcting code. However, these recommended measures proved to be little effective when error packets occur whose duration exceeds the duration of the synchronizing word and whose appearance more or less coincides with this word. The synchronizing word is then cancelled and it cannot perform its part any more.